In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines.
The requirement of small features with close spacing between adjacent features requires high resolution lithographic processes. In general, projection lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through a transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. A recognized way of reducing the feature size of circuit elements is to lithographically image them with radiation of a shorter wavelength. "Long" or "soft" x-rays (a.k.a, extreme ultraviolet (EUV)), wavelength range of lambda=50 to 700 Angstroms (.ANG.) are now at the forefront of research in an effort to achieve the smaller desired feature sizes.
EUV lithography may be carried out as follows, EUV radiation is projected onto a resonant-reflective reticle. The resonant-reflective reticle reflects a substantial portion of the EUV radiation which carries an IC pattern formed on the reticle to an all resonant-reflective imaging system (e.g., series of high precision mirrors). A demagnified image of the reticle pattern is projected onto a resist coated wafer. The entire reticle pattern is exposed onto the wafer by synchronously scanning the mask and the wafer (i.e., a step-and-scan exposure).
Although EUV lithography provides substantial advantages with respect to achieving high resolution patterning, errors may still result from the EUV lithography process. For instance, the reflective reticle employed in the EUV lithographic process is not completely reflective and consequently will absorb some of the EUV radiation. The absorbed EUV radiation results in heating of the reticle. As the reticle increases in temperature, mechanical distortion of the reticle may result due to thermal expansion of the reticle. Such mechanical distortion of the reticle manifests in overlay errors. In photolithography, overlay is defined as layer to layer registration performance. For example, silicon is a material which may be used as a substrate for a reticle, and silicon has a coefficient of thermal expansion of approximately 2 ppm/.degree.C. Across a typical 100 mm image field, a 0.5.degree. C. deviation in temperature will result in a registration error of 100 nm, in circumstances where &lt;10 nm is desired.
Thus, in order to mitigate overlay error it is desirable to prevent heating of the reticle. Transferring heat away from the reticle using fluids is not satisfactory because the use of cooling fluids is not practicable when considering the need to control precise positioning of the reticle (e.g., &lt;10 nm) as it is scanned dynamically at speeds which may exceed one meter per second. Conventional radiative transfer and conduction cooling is typically inadequate because the temperature of the reticle is too close to ambient temperature, which runs contrary to radiative transfer and conduction cooling. Consequently, there is a need for a means to prevent the reticle from heating to a level where mechanical distortion may occur in order to mitigate overlay error.